1. Field of the Invention
The present invention relates to a backside-illuminated solid-state image pickup device and a manufacturing method thereof.
2. Description of the Related Art
The market for small-sized camera modules has been drawing attention in recent years with application of these modules to digital still cameras or camera-equipped mobile telephones. With a size reduction of pixels, an image device such as a CCD or a CMOS image sensor used for such a camera module may have lower sensitivity reduced due to a reduction in a photoelectric conversion area. The factors of the sensitivity reduction include the reduction in the photoelectric conversion area due to smaller pixels, and a reduction in the amount of incident light attributable to shielding of light by wiring.
To cope with this, a backside-illuminated image pickup device has been reported as a technique for improving sensitivity and avoiding reduction in the amount of incident light. Here, a conventional image-pickup device will be referred to as a front side-illuminated image pickup device in contrast to this backside-illuminated image pickup device.
The backside-illuminated image pickup device has peripheral circuits such as a logic circuit and an analog circuit which are formed on a front side of a silicon substrate, and a photoelectric conversion area, color filters, microlenses, and the like which are formed on a backside of the silicon substrate. The light incident on the backside of the silicon substrate is subjected to photoelectric conversion inside the silicon substrate. The electric signal thus obtained is subjected to signal processing on the front side of the silicon substrate, and then is outputted from the image pickup device.
This device achieves high sensitivity and low optical shading since no optical obstacle such as wiring exists between the microlenses and photoelectric conversion area on the backside of the silicon substrate which receives incident light. Moreover, the device also achieves high light focusing performance and low crosstalk because the color filters and the microlenses can be formed with low profiles.
However, the backside-illuminated image pickup device has the following problem related to grounding. Specifically, the conventional front side-illuminated image pickup device can be grounded with the back surface of the silicon substrate bonded to a package, and thereby the analog circuit and the like can produce stable waveforms. In contrast, having the microlenses and the like on the backside of the silicon substrate, the backside-illuminated image pickup device cannot be grounded with the back surface of the silicon substrate bonded to a package.
As a countermeasure against the problem, there is disclosed a backside-illuminated image pickup device which draws out minority carriers generated in photoelectric conversion elements in a pixel area and thereby suppresses reduction in sensitivity by grounding a photo-shielding layer that is formed around the pixel area (Japanese Patent Application Publication No. 2006-32497 (Paragraph [0020], FIG. 2), for example).
However, this image pickup device is configured without consideration of peripheral circuits including a logic circuit and an analog circuit. Accordingly, in this device, the analog circuit may fail to produce stable waveforms due to a delay or unsharp edges of a reference waveform, or pulse noises may be likely to leak in the logic circuit. This causes a problem of defects on a reproduced image that appear as vertical lines or horizontal lines, for example.